The inter-application communication in a main system uses TCP (Transmission Control Protocol) that has a retransmission control function and the like, since high reliability is required. In addition, an active device and a standby device are provided so that the process can be carried on when a failure occurs in a device.
In the above system, TCP connection information (the IP address, port number, sequence number) of the active device and data to be transmitted need to be passed on to the standby device when a failure occurs, but the passing on has been difficult for the following reasons.    (1) The passing on of the IP address and port number can be passed on by using a common IP address and port number between the two devices, but secure passing on of the sequence number and transmission data is difficult, since they change in packets that flow at any time during the communication.    (2) The TCP connection is point-to-point connection where there are one-on-one transmitter and recipient, so it is not allowed in the TCP to establish connection using the same TCP connection information simultaneously between a plurality of devices.    (3) As a method for solving the above problems, it is possible in theory to transmit, with every packet transmission/reception, information including its sequence number and data from the transmitter to a standby device, and to perform, after that, notification of the received data to an upper application and transmission of the transmission data to the destination apparatus. However, such implementation is difficult, since the transfer performance of the TCP deteriorate significantly with this method, and the implementation layer of the TCP is OS (kernel).
Therefore, under the current situation, it is required to switch to a standby device at the time when a failure occurs in the active device and after that, to establish the TCP connection again between the client and the standby device, and then the operation is resumed after performing confirmation of the delivery of data between the applications. For this reason, there has been a problem that it results in a long disconnected time of the operation.
While it is possible to transmit the same data simultaneously to a plurality of destinations by using the multicast communication function of UDP (User Datagram Protocol) the UDP does not have reliability functions such as the delivery confirmation, retransmission control, and so on, it cannot be used for the inter-application communication in a main system.
To implement a connection control function uniquely in the application itself and to manage its connection with a plurality of communication destination devices to secure the continuity of the operation with the switch to a standby device when a failure occurs is considered as another method.
However, it is required with the method to equip each application with a connection control function and a function to manage the connection with a plurality of communication destination devices, necessitating a significant change to the process at the application side.
Patent document 1 describes equipping an application with a function to transmit confirmation response information in response to a plurality of data packets using a single response packet, to reduce the band used for the transmission of the confirmation response packet.
Patent document 1 does not describe performing delivery confirmation of a data packet in multicast communication.
Patent document 2 describes transmitting, from a server, a packet while attaching, to each packet, representative identification information that specifies a representative client, and transmitting a response packet from only the client that received the packet having the representative identification information, to reduce the number of response packets.
Patent document 2 does not describe securing of reliability that equals to that of the TCP in multicast communication.    Patent document 1: Japanese Laid-open Patent Application No. 2004-80070    Patent document 2: Japanese Laid-open Patent Application No. 2004-120137